1. Field of the Invention
This invention relates in general to drilling and production of oil and gas wells and, in particular, to a tubular connector having a secondary load transfer shoulder.
2. Brief Description of Related Art
Offshore hydrocarbon wells often contain one or more casing strings of large diameter pipe, such as 16 inches or greater in diameter. Production risers extending from subsea well equipment to the surface are also made up of fairly large diameter pipes. The joints of pipe, whether in a casing string or production riser string, are connected together by threaded connections. A typical threaded connection has internal threads on a conical portion of a box that engage external threads on a pin. Normally, the pin has an external shoulder at the base of the threads that is engaged by the rim of the box when made up. A compressive load on the string of casing acts against the external shoulder. Typically, the compressive load capacity of the connector is less than the compressive load capacity of the pipe.
Connectors having both external and internal load shoulders are known in the prior art. The nose of the pin engages the internal load shoulder and the rim of the box engages the external load shoulder. However, tolerance stack ups and high stiffness make it difficult to achieve proper engagement of both the internal and external shoulders. Under compressive loading, the outer diameter shoulder also creates an internal bending moment that is offset from the compressive load path transmitting through the pipe to the connector and back into the pipe. The bending moment contributes to the connectors having a compressive load strength that is less than the compressive load strength of the tubular members individually. Internal shoulders alone create very eccentric paths for tension loads and very high stresses under combined compression and internal pressure. As a result, the connectors are often formed of stronger materials or have larger diameters to accommodate the additional compressive load. This can make it difficult to manufacture the tubulars, add to their weight, and make manipulating the tubulars more difficult. In addition, where the joined tubulars are run concentrically through another set of tubulars, the outer tubulars must be increased in size to accommodate the larger diameter of the joints.
Attempts have been made to place dual load transfer shoulders on both the internal and external diameters of the connectors to increase the load transfer area and counteract the bending moment. However, this resulted in little control of where the load transfers due to tolerance stack up issues and high stiffness of the internal shoulder.